For \(\rho _s>\rho _{ps}\) , \(p(\rho _s)\)  is nonzero and the leading order
of the divergence in \(f_0\)  is \(z^{-1/2}\) , which have a finite
result. As \(\rho _s\longrightarrow \rho _{ps }\) , \(p(\rho _s)\)  approaches
zero and divergence is of order \(z^{-1}\) , that makes the integral
divergent. Therefor, the deflection angle can be approximated in the
following form [1]},
\(\alpha =-\bar{a}\,log\left(\frac{u}{u_{sp}}-1\right)+\bar{b}+O(u-u_{sp}),\) 
